DNA, the blueprint of life located inside our cells, is under continuous assault by both environmental and intracellular DNA damaging agents. If not corrected, these errors may lead to the development of cancer. Fortunately, cells have built up elegant DNA surveillance networks, termed cell cycle checkpoints, to detect and repair damaged DNA. Central to cell cycle checkpoints is a protein kinase, Chk1, which is essential for cell viability and mammalian embryonic development. Intriguingly, increasing evidence suggests that Chk1 may benefit tumor growth as well, as Chk1 expression is increased in malignant tumors and its level positively correlates with tumor grade. Most importantly, we recently showed that Chk1 is centrally involved in the resistance of cancer cells to chemotherapeutic drugs. These findings highlight the importance of Chk1 in cancer etiology and therapy. Therefore, a better understanding of Chk1 regulation not only provides insights the DNA damage response, but may also lead to the development of novel strategies in cancer therapy. Recent studies from this laboratory and others revealed an exciting spatiotemporal regulation model of Chk1, in which DNA damage induces phosphorylation and a rapid release of Chk1 from chromatin into the nucleus, and subsequently into the cytoplasm. Failure to undergo chromatin release of Chk1 impairs cell cycle checkpoints. On the other hand, cellular accumulation of active Chk1 proteins leads to the resistance of cancer cells to a clinically used chemotherapeutic drug, camptothecin (CPT). This is probably because Chk1-profient cells are better equipped to handle DNA damage induced by CPT. Thus, a general hypothesis is that the dynamic mobilization of Chk1 from one cellular compartment to another is crucial for the activation of cell cycle checkpoints in response to DNA damage, as well as for the chemotherapy response. In order to test this hypothesis, we first wish to understand the detailed molecular mechanisms underlying this chromatin->nucleus->cytoplasm mobilization of Chk1 in cells. We will first determine the factors that regulate chromatin association and disassociation of Chk1, a non-DNA binding protein (Aim 1). Second, we will define how Chk1 is exported from the nucleus to the cytoplasm (Aim 2). Further, we will interrogate the functional significance of the spatiotemporal regulation of Chk1. To do so, we will utilize a novel in vitro system established by this laboratory that highly recapitulates cell cycle checkpoints, as well as unique cell lines we generated with Chk1 mutant proteins specifically localized in a particular cellular compartment (Aim 2). Lastly, we will expand this study from a pure biochemical and molecular research into something that will offer novel insights into cancer etiology and chemotherapy through testing two hypotheses: one is that blocking the spatiotemporal mobilization of Chk1 sensitizes cancer cells to chemotherapeutic drugs, and the other is that Chk1 is a predictive marker for the therapy response of human tumors to chemotherapy (Aim 3).